This invention relates to a propulsion unit of a marine vessel.
In this specification the terms “after,” “behind,” “rear,” “in front of” and similar expressions, used in connection with describing a location of an element relative to a marine vessel, are to be understood in the context of the bow of the vessel being considered the front of the vessel and the stern of the vessel being considered the rear of the vessel.
Propulsion of a ship has traditionally been accomplished by using one or several rotating propellers and steering has been accomplished by making use of one or more rudders in connection with a propeller. A contra rotating propeller (CRP) system is known to offer some benefits over a conventional single propeller. For instance, in the case of a CRP system the load is distributed onto the blade area of two propellers instead of one, and the rotational energy of the slip stream of the first propeller may be recovered more efficiently, just to mention two benefits.
The most common solution of providing a CRP system employs two concentric shafts, i.e. a hollow outer shaft and an inner shaft inside the outer shaft. In this kind of solution the first, or front, propeller is connected to the outer shaft and the second, or rear, propeller is connected to the inner shaft, which extends through the hub of the first propeller. U.S. Pat. No. 4,963,108 may be regarded as an example of this kind of a solution. This arrangement requires rather complicated shafting and gear devices. In practice also sealing is prone to be problematic to arrange.
As an alternative, a combination of a traditional shaft driven single propeller and a CRP pod as rear propeller has also been suggested. In this case, the CRP pod is used to steer the vessel and there is no separate rudder. With this solution it is possible avoid the complicated shafting and gear devices of the conventional CRP system having two concentric shafts. However, this solution requires a relatively large gap between the propellers, because the pod must be able to turn behind the front propeller. This in turn decreases the efficiency of the solution.
It is an object of the invention to provide a propulsion unit of a marine vessel, which minimizes the problems of the prior art. It is a specific object of the invention to provide a propulsion unit of a marine vessel, which is additionally capable of steering action.